TUESDAY, Nov. 13 (HealthDay News) -- For the fifth year in a row, the preterm birth rate in the United States has dropped. The 2011 rate was the lowest in 10 years, the March of Dimes reported today.

But, there's still significant room for improvement. Nearly half a million babies in the United States are still born prematurely, noted the March of Dimes in its 2012 Premature Birth Report Card. Overall, the U.S. grade remains a "C" because of the continued high preterm birth rate.

A preterm birth occurs at less than 37 weeks of pregnancy. Prematurity puts infants at risk for a number of health problems including breathing difficulty, heart defects and bleeding in the brain. Some conditions are only temporary while others can persist.

In 2011, "the rate of preterm birth [was] at 11.7 percent, which is the lowest we've seen in a decade," said Dr. Edward McCabe, medical director for the March of Dimes. "That means 64,000 fewer babies were born prematurely in 2010 compared to 2006, the peak year for preterm birth. Along with the personal cost, there's also an economic cost, and there's a potential savings of $3 billion in health care and economic costs associated with those 64,000 babies not being born preterm."

The goal, McCabe said, is to get the number of preterm births down to 9.6 percent by 2020.

Four states -- Maine, New Hampshire, Oregon and Vermont -- have already achieved that goal, and were graded an "A" by the March of Dimes. Twenty-two states were closing in on the goal and received a "B" grade. Almost all states -- 45 -- improved between 2009 and 2011. However, just 16 of them improved enough to increase their letter grade.

Three states -- Alabama, Louisiana, Mississippi -- and Puerto Rico received a failing grade. Alabama's preterm rate was 14.9 percent, Louisiana's was 15.6 percent, Mississippi's was 16.9 percent and Puerto Rico had a preterm birth rate of 17.6 percent.

Several major factors that still contribute to preterm births, the March of Dimes says, are maternal smoking, a lack of health insurance and early induction of labor or a scheduled cesarean delivery between 34 and 36 weeks of gestation without a medical reason for the early delivery.

Delivery between 34 and 36 weeks is considered late preterm birth.

Four states had maternal smoking rates above 30 percent, with West Virginia leading the pack with nearly 36 percent of expectant moms still smoking. The other three states with significantly high rates were Kentucky, Ohio and South Dakota.

Texas had the worst rates of uninsured women with 34.2 percent without insurance. Florida and New Mexico had rates of uninsured women of about 30 percent.

The March of Dimes also looked at late preterm birth rates.

Just three states and Puerto Rico had double-digit rates of late preterm birth. Puerto Rico's rate was the highest at 12.6 percent followed by Mississippi at 11.9 percent. Louisiana's rate was 10.6 percent followed by Alabama with 10.3 percent, according to the report card. McCabe said 39 weeks or later is considered a full-term birth.

To try to further reduce the rate of preterm birth, 48 states along with the District of Columbia and Puerto Rico have pledged to work with the March of Dimes to reduce their preterm birth rates by 8 percent by 2014.

On an individual level, women can make changes that can help reduce the rate of preterm birth, too, such as quitting smoking and eating a healthy diet, McCabe said. He said it's also important to have a preconception visit with your doctor if possible, and to go to every prenatal visit, even if you're feeling fine.

Another expert, Dr. Deborah Campbell, director of the division of neonatology at Montefiore Medical Center in New York City, said that while the lower rates are good news, more needs to be done.

"States that have made progress should applaud that progress, but when you look at the map, there are still many states that need to refocus and redouble their efforts," Campbell said.

Campbell agreed that the three factors singled out by the March of Dimes are important, but added that "there are a lot of other factors we don't have a good handle on. The rates of preterm birth for babies less than 32 weeks have stayed steady for about two decades. For these tiniest of babies, we still don't have a full understanding of how to prevent preterm birth."